Information processing was monitored by averaged evoked response techniques. The electrographic activity was recorded from left and right brain regions during memory and perception in normal subjects, patients with unilateral temporal lobectomy, and patients with neuropsychiatric disorders. Electroencephalographic disturbances in brain-behavior relations in psychiatric patients were also evaluated, relating left and right brain dysfunctioning to activity to maladaptive ideative and emotional reactions, respectively. In temporal lobectomy patients, P300 amplitude was found to be inversely proportional to stimulus probability in the same way as for normal controls, and larger P300s were elicited in reaction time. For visual material, right-temporal patients manifested smaller P300s at frontal sites than either left-temporal or normal individuals. Moreover, there were no consistent hemispheric asymmetries which distinguished the left- or right-temporal patients, or either group from normal subjects. These data discount the hypothesis that medial temporal structures, including the hippocampus, serve as a sole generator of P300. More specifically the data indicate that processing of auditory and visual stimuli is dependent to a great extent on the character of the material and the integrity of left and right brain mechanisms.